In recent years, so as to speed up image processing of liquid crystal display apparatuses and improve display image quality, so-called active matrix type liquid crystal display apparatuses where switching thin film transistors (hereinafter referred to as TFTs) are disposed corresponding to display picture elements (pixels) have been developed.
Switching TFTs that contain amorphous silicon (a-Si) or poly silicon (poly-Si) have been widely used for such active matrix type liquid crystal display apparatuses.
In particular, poly-Si TFTs have high mobility and is incorporated as driver circuits with switching TFTs corresponding to pixels on the same substrate from the process coordination point of view. Thus, it is known that poly-Si is a constructional material of TFTs suitable for active matrix type liquid crystal display apparatuses, which should be small and have precise display characteristics.
Next, a construction of a switching element array substrate for use in a conventional active matrix type liquid crystal display apparatus will be described in brief. FIG. 9 is a plan view showing a switching element array substrate. FIG. 10 is a sectional view (taken along line A-A' of FIG. 9) chiefly showing signal lines of a portion covered with a sealing member. FIG. 11 is a sectional view (taken along line B-B' of FIG. 9) chiefly showing scanning lines of a portion covered with the sealing member. For simplicity, FIG. 9 shows only nine pixels in the display region of a liquid crystal display panel. In addition to constructional elements shown in FIG. 9, storage capacitors Cs, storage capacitor lines coupled thereto, and the like are formed on a TFT substrate. For simplicity, these capacitors, lines, and the like are omitted.
As shown in FIG. 9, TFTs 503 that are used as switching elements of a pixel portion each are formed on a switching element array substrate 501 of the liquid crystal display apparatus. A drain of each TFT 503 is connected to a signal line 505. A source of each TFT 503 is connected to a pixel electrode 507. A gate of each TFT 503 is connected to a scanning line 509.
The signal line 505 extends outside a display region through a sealing member 511 and is connected to signal line driver circuits 513. The scanning line 509 extends outside the display region through the sealing member 511 and is connected to scanning line driver circuits 515.
As shown in FIG. 10, which is a sectional view taken along line A-A' of FIG. 9, the layer insulating film 601, the signal line 505, and the protecting film 603 are layered on the glass substrate 600. The layer insulating film 601 is made of for example SiO.sub.x. The signal line 505 is formed by patterning a Al/Cr film on the glass substrate 600.
On the other hand, as shown in FIG. 11, which is a sectional view taken along line B-B' of FIG. 9, the scanning line 509 is formed by patterning a poly-silicon film on the glass substrate 600. Since the poly-silicon film contains impurities, the resistance of the poly-silicon film is low.
The switching element array substrate 501 and an opposed substrate (not shown) are positioned face to face. The opposed substrate has a counter electrode and aligning film. The counter electrode is made of ITO. The sealing member (that serves both as a sealing member and a bonding agent) 511 is printed or coated with a predetermined width over the signal lines 505 that extend between the signal line driver circuits 513 and the display region, and over the scanning lines 509 that extend between the scanning line driver circuits 515 and the display region. Both of the substrates are facing each other with a gap and bonded with pressing so that the aligning directions of the aligning films of the substrates are perpendicular to each other. A cavity liquid crystal cell defined by the gap between the two substrates and the sealing member is filled with a liquid crystal composition (not shown) that serves as an optical modulating layer. In this manner, a liquid crystal display apparatus is formed.
The sealing member 511, which bonds the above- mentioned two substrates, is formed by mixing a spacer with a bonding agent. The spacer is made of for example a gap controlling member with the same diameter and length as the gap to be kept for substrates. For example, the spacer is made of needle-like glass fibers with a diameter of 5 .mu.m and a length of 20 .mu.m to 200 .mu.m. The sealing member is printed or coated over the above-mentioned region so as to keep the gap between the two substrates equal.
However, at the portion of each signal line 505, since the layer insulating film (0.45 .mu.m), the Al/Cr dual layer (0.8 .mu.m/0.05 .mu.m) and the protecting film (0.3 .mu.m) are formed, the thickness measured from the front surface of the substrate 600 covered with the sealing member 511 of the switching element array substrate 501 to the protecting film 603 (namely, the gap between the substrates) is 1.6 .mu.m. On the other hand, at the portion of the scanning line 509, since the poly-silicon film (0.4 .mu.m), the layer insulating film (0.45 .mu.m), and the protecting film (0.3 .mu.m) are formed, the thickness measured from the front surface of the substrate 600 covered with the sealing member 511 of the switching element array substrate 501 to the protecting film 603 is 1.15 .mu.m. The difference of thickness of these portions is 1.6 .mu.m-1.15 .mu.m=0.45 .mu.m. In other words, the difference of gaps of the substrates held by the sealing member 511 is approximately as many as 10% of 5 .mu.m that is the gap between the substrates. The difference of thicknesses of the sealing member 511 at the two portions results in deviation on a screen formed by the gap between the two substrates. Thus, a display image on the screen (display region) becomes uneven. In particular, the substrate gap in the vertical direction of the screen does not match the substrate gap in the horizontal direction of the screen. Thus, the two substrates are not positioned in parallel with each other. Therefore, an uneven display image takes place.
As described above, the difference between the height of the sealing member over the scanning lines 509 and the height of the sealing member over the signal lines 505 results from the difference of layer construction and difference of film thickness. However, to form TFTs, the layer construction of the scanning lines 509 should be different from the layer construction of the signal lines 505. In other words, a layer on which the scanning lines 509 are formed is different from a layer on which the signal lines 505 are formed. The layer of the scanning lines 509 and the layer of the signal lines 505 are insulated by a layer insulating film 601 or the like. In each TFT 503, the gate (or gate electrode) is connected to the scanning line 509. The drain (or drain electrode) is connected to the signal line 505. The gate and drain of the TFT 503 are formed on different layers through a semiconductor layer. The process conditions for these layers are remarkably different from each other regardless of whether the gate electrode is disposed over the semiconductor layer (namely, stagger construction) or below the semiconductor layer (namely, antistagger construction). Thus, the scanning lines 509 and the signal lines 505 should be formed on different layers with different thicknesses. The thickness of the portion of the scanning lines 509 on which the sealing member is formed is necessarily different from the thickness of the portion of the signal lines 505 on which the sealing member is formed. In addition, the thicknesses of these portions vary in their fabrication processes.
The length of the glass fibers is around 20 .mu.m to 200 .mu.m and the end surfaces thereof are sharp. Thus, when the glass fibers are positioned over the signal lines 505 and the scanning lines 509 and bonded thereto with pressing, the glass fibers occasionally damage the signal lines 505 and the scanning lines 509.
To obtain a high quality display image, color and contrast should be improved. The uneven colors take place due to coherence of light through liquid crystal display panel because of unequal gap between two substrates of the panel. In addition, difference of electric fields applied to liquid crystal composition and difference of retardation result in uneven contrast and uneven colors. To obtain a gray scale display image and a high resolution display image, a liquid crystal composition that has a quick voltage-response characteristic is used. The light transmittivity of a liquid crystal composition with a quick voltage-response characteristic largely varies as the retardation varies. Thus, when the gap between the substrates is uneven, the retardation largely varies, thereby remarkably lowering the evenness of intensity of display image, position by position. In particular, in the case of a projection type liquid crystal display apparatus with three liquid crystal display devices, such a problem becomes important.
For a display image quality of such a liquid crystal apparatus, in reality, the deviation of gap between two substrates should be decreased to around 0.1 .mu.m or less. To accomplish high quality display image, factors that cause an image to be uneven other than errors in fabrication process should be removed.
As described above, since the substrate gap at the portion where the scanning lines are covered with the sealing member (namely, the thickness measured from the front surface of the substrate to the upper surface of the protecting film over the scanning lines) differs from the substrate gap at the portion where the signal lines are covered with the sealing member (thickness measured from the front surface of the substrate to the upper surface of the protecting film over the signal lines), uneven colors, uneven contrast, and so forth take place in a display image, thereby deteriorating display image characteristics.
The present invention has been made to solve such problems. An object of the present invention is to provide a liquid crystal display apparatus with an equal gap between two substrates so as to improve display image quality and display image contrast.